1. Field of the Invention
This invention relates to a picture information transmission system and picture coding and decoding apparatus for use with the picture information transmission system, and particularly to a picture information transmission system which codes a moving picture based on temporal correlation (interframing coding) and enciphers and transmits the coded picture.
2. Description of the Related Art
In picture information transmission systems which code picture information based on interframe frame coding and encipher and transmit the coded picture information, it is generally impossible to control switching between proceeding and stopping of encipherment or, even if this is possible, the switching is only possible at the end of long time intervals. For example, in transmission of a moving picture formed of a large number of successive frames, proceeding/stopping of encipherment cannot be switched for a minimum of at least about two hours.
Therefore, a receiver apparatus which does not have a cipher decoding device cannot effect decoding and therefore, cannot display any part of a correct picture while encipherment is proceeding on the transmission side (in the example described above, for a minimum of at least about two hours).